@Lotrei Presumably Kosuzu is an M and / or making an "it's okay if it's you" justification due to pre-existing feelings for Mamizou.
To my understanding, that last page explains that it's a twisted sort of love on both of their parts. Kosuzu giving herself up to essentially be Mamizou's slave is how she expresses that love, and Mamizou indulging her is how she expresses it.
Like the last page says, love can be terrifying. Especially when the parties involved express it in unconventional ways